In the development of personal computers, two different basic drive configurations have been established. One of these is the SCSI drive used with IBM.RTM. compatible personal computers. The SCSI drive is 16 bits wide. A different drive, which is known as the IDE drive, is utilized in conjunction with Apple Computer's MacIntosh.RTM.. IDE drives typically are 8 bits wide; although various versions of IDE drives exist which are 16 bits wide. Common hardware reference platforms for IDE drives specify an 8-bit wide drive with parity interface.
Peripheral equipment employing 32-bit wide data transfer is coupled with either of the above general computer types through what is known as a PCI interface. The PCI 32-bit wide drive is widely used, and therefore requires an interface between the IDE or SCSI drives to convert the PCI 32-bit wide data to either the 16-bit wide SCSI output or 8-bit wide IDE drive configuration, and vice versa. In the past, two separate interface chips have been required for interfacing between the respective IDE or SCSI drive and a PCI bus interface. This required manufacturers of such interfaces to build two different sets of interfaces and to maintain inventories of such different sets for filling orders with the respective IDE or SCSI computer configurations.
To eliminate the necessity for maintaining inventories of two different types of disk controller interfaces, interfaces have been built with two separate chips, one dedicated to IDE interfaces and the other dedicated to SCSI interfaces, with only one of the circuits subsequently being used and being programmed at the time the interface was shipped to the customer (either IDE or SCSI). This caused one of the circuits on the interface to be unused, since the systems could not be switched between one or the other. The output pins were not common. One set was for interconnection with IDE drives; and the other set was for connection with SCSI drives. This resulted in a relatively expensive and wasteful configuration of common hardware platforms.
Because of the incompatibility of Apple Computer (IDE) and IBM compatible computer (SCSI) platforms, common hardware platforms for implementation with either of these types of systems have not been considered possible. As a consequence, system designers were required to carefully select disk controller systems designed specifically for driver support for the choice of interface for the operating systems based upon the computer platform used.
It is desirable to provide a combined IDE and SCSI disk controller for common hardware platforms interfacing with a PCI bus which is cost effective and which shares common components. An example of a platform that requires both IDE and SCSI drives is the CHRP, common hardware reference platform, as defined by the Power PC Microprocessor.